With known virtual image displays, a user does not view directly a physical display screen such as with real image displays. Typically, the virtual display creates only a small physical image using a liquid crystal array, light emitting diodes or a miniature cathode ray tube, CRT, the image being projected by optical lenses and mirrors so that the image appears to be a large picture suspended in the world.
A miniature cathode ray tube can produce a medium resolution monochrome picture. However, these devices are heavy and bulky. For example, a typical weight of a miniature CRT with cables is greater than four ounces, the CRT having a one inch diameter and a four inch length. Further, these devices have high voltage acceleration potential, typically 7-13 kilovolts which is undesirably high for a display that is mounted on a user's head. Creating color using a single miniature CRT is difficult and usually causes significant compromises in image resolution and luminance. Although the CRT image may be relayed via a coherent fiber-optics bundle to allow the CRT to be located away from head mounted optics, the hardware to accomplish this is also heavy and causes significant light loss. Field sequential color using a multiplexed color filter and CRT with white phosphor is able to create good color hue saturation but also at a significantly reduced resolution. For example, three color fields must be produced during the same period as a normal 60 Hz field, thereby dividing the video bandwidth for each color by three.
A liquid crystal array can produce a color image using a low operating voltage, but it can provide only a marginal picture element (pixel) density, i.e. less than 800 by 800 elements. One commercial device is known that uses a linear array of light emitting diodes viewed via a vibrating mirror and a simple magnifier. Although this is a low cost and low power alternative, the display is monochrome and limited in line resolution to the number of elements which can be incorporated into the linear array.
Both the CRT and liquid crystal display generate real images which are relayed to the eyes through an infinity optical system. The simplest optical system allows a user to view the image source through a simple magnifier lens. For fields of view greater than 30.degree., this approach leads to a number of problems including light loss and chromatic aberrations. Further, these optics are bulky and heavy.
Virtual projection optical designs create an aerial image somewhere in the optical path at an image plane which is then viewed as an erect virtual image via an eye piece or objective lens. This approach increases the flexibility by which the image from the image source can be folded around the user's head for a head mounted display system, but large fields of view require large and bulky reflective and refractive optical elements.
In addition to resolution limitations, current systems also have bandwidth deficiencies. Bandwidth is a measure of how fast the display system can address, modulate or change the light emissions of the display elements of the image source. The bandwidth of the display image source is computed on the basis of the number of elements which must be addressed over a given period of time. Addressing elements temporally is needed to refresh or maintain a perceived luminance of each element taking into account the light integration dynamics of retinal receptors and the rate at which information is likely to change. The minimum refresh rate is a function of the light adaptive state of the eye, display luminance, and pixel persistence, i.e. the length of time the picture element produces light after it has been addressed. Minimum refresh rates of 50 to 60 times a second are typically needed for television type displays. Further, an update rate of at least 30 Hz is needed to perceive continuous movement in a dynamic display or in a presentation in which the display image is stabilized as a result of head movement. Refreshing sequentially, i.e. one element at a time, 40 million picture elements at a 60 Mhz rate would require a video bandwidth of 2.4 GHz. Bandwidth requirements can be reduced by interlacing which tricks the eye in its perception of flicker but still requires that all of the elements of the image source be addressed to achieve a minimum update rate of 30 Hz or 1.2 GHz bandwidth. Typical television broadcast quality bandwidths are approximately 8 MHz, or two orders of magnitude less than the 1.2 GHz. High resolution computer terminals have 1400 by 1100 picture elements which are addressed at a 70 Hz non-interlaced rate which is the equivalent to a bandwidth of approximately 100 MHz.